I am one of those who likes to try "bleeding edge" stuff, that's why I run gentoo, and therefore I try to run 2.5.x kernels. The url in this report has patches for nvidia-kernel 3xxx/4xxx on 2.5 kernels. An option should be to have an environment variable or useflag for building nvidia-kernel for 2.5 kernels. Also, there should be support for the new depmod requirement of 2.5.49+.
Looking in the ebuild, the patches are applied.